Organizers said the event will be rescheduled for September or October. The memorial, the first on the Mall honoring an African American, has been a quarter-century in the making, but safety trumped ceremony.

Hurricane Irene was forecast to sweep over the Outer Banks of North Carolina overnight Friday and advance into the Washington area with a vanguard of showers beginning Saturday afternoon.

Early Friday morning, the National Weather Service upgraded the Tropical Storm Watch issued for much of the D.C. area to a Tropical Storm Warning. Meanwhile, Irene weakened slightly to a Category 2 storm as it approached the East Coast, where a hurricane warning was also extended to New Jersey.

If the hurricane stays on track, the worst of Irene will arrive in Virginia, Maryland and the District later Saturday and into Sunday morning. Late-summer vacationers evacuated Atlantic coast beaches, which are expected to be hit hardest before the storm wallops New England.

The intensity of the storm and the shift in the forecast track farther to the west prompted the decision to delay the memorial dedication, said Harry E. Johnson Sr., chief executive of the memorial project foundation.

“I’m disappointed and hurt, really,” Johnson said. “But the memorial is going to be there forever.”

Johnson said the change might allow those who planned to travel to stay home and for those in Washington to leave ahead of the storm.

Governors along the coast, including those in Virginia and Maryland, declared states of emergency Thursday, and thousands of weekend events were canceled.

“This is a large, this is a deadly, this is a slow-moving hurricane that is bearing down on the state of Maryland,” Maryland Gov. Martin O’Malley (D) said in declaring an emergency. “There will no doubt be a lot of flooding. Citizens should anticipate long periods of electrical outages.”

A significant storm surge is expected to flood coastal areas, and wind-driven flooding may occur along the shores of the Chesapeake Bay. The worst of the weather is likely to be east of the Interstate 95 corridor, which may get four to six inches of rain, prolonged winds of 50 to 70 mph, and gusts of 90 to 100 mph, according to meteorologists with The Washington Post’s Capital Weather Gang.

An endless stream of vacationers rolled across the bridge out of Ocean City, on Thursday evening, and homeowners rushed in the opposite direction to board up their rental properties. Ocean City was one of many resort areas where evacuation was mandatory.

Colleges on the verge of opening for the fall semester warned students to delay their arrival, and the College of William and Mary in Williamsburg told its students to go home.

Three other schools — the University of Maryland, George Washington University and Catholic University — said they would open their dormitories a day early, on Friday, so that students could get settled before the storm hit. George Mason University said it would implement a flexible move-in schedule.

In New York, the Associated Press reported that Mayor Michael R. Bloomberg (I) said officials expect to shut down the city’s transit system Saturday afternoon ahead of the hurricane, which is forecast to strike eastern Queens.

After passing over the Bahamas on Thursday, the storm first fell on the U.S. coast in Florida, where its outermost bands swept in with bursts of wind and rain and a driving riptide.

The exodus from the Outer Banks began early Thursday after an evacuation order Wednesday night. Traffic on Route 168 crawled as lines of sport-utility vehicles with surfboards and fishing rods mounted on their roofs headed north from the barrier islands.

“My aunt and uncle are used to storms, but they got a bit worried about this one,” said Melissa Wallace of St. Louis, who had been vacationing at their Cape Hatteras beach house. “We just thought better safe than sorry.”

In the Washington region, there were warnings that people should be prepared for power outages as toppling trees take down electrical lines. Road crews were on alert to clear fallen trees and other wind-driven debris from highways.

More than 2,000 sandbags were being placed at Metro stations where water tends to come up over the curbs and flow down escalators. Metro crews also checked drains in tunnels, and some vehicles assigned to Metro supervisors were being equipped with chain saws to keep the transit system moving. The District and Alexandria offered free sandbags to residents.

O’Malley said the mandatory evacuation of Ocean City underscored the seriousness of the storm. “This is not a time to get out the camera and sit on the beach and take pictures of the waves,” he said.

“I reserve the right to direct and compel evacuation from the same and different areas and determine a different timetable both where local governing bodies have made such a determination and where local governing bodies have not made such a determination,” McDonnell said in a statement.

Pepco urged customers who need power for critical medical equipment to review emergency plans and be prepared for extended power outages. Dominion Virginia Power and BGE said repair crews were preparing for emergency restoration work over the next several days. Extra crews from other states were headed to the region to assist with recovery.

“This storm has serious potential to cause widespread damage,” said Rodney Blevins, Dominion’s vice president. “We are geared up to handle any situation as quickly and safely as possible. We are treating Hurricane Irene seriously, and we urge our customers to monitor local weather forecasts for changing conditions in order to remain safe.”